Business plan??? Doesn't he already have a "business plan"? Start small, work hard, gradually build up the business until he can quit his day job.
If that's all the OP as these days, his chances of success are slim.
40 years ago, the world was an entirely different place. You were still stuffing cassettes in your car.
OP needs to sit down and get an accurate (or as close as he can) estimate of all expenses to run that shop.
Can he operate the shop on his current income without affecting the household?
Baby on the way, increase in household expenses for sure.
We don't know his financial setup so that plays a part in this too.
Biggest thing he needs to decide is he willing to sacrifice family to achieve this goal.
Family will suffer, there is no way around it. Soon as he takes it on his mindset will change.
Worried about income, getting parts done etc etc
It's just the way it is....he has to decide if it's worth it in the end.
OP may luck out and fall into jobs and all is good, or he could work his butt off, have a lousy family life, no time for any family vacations, have his wife divorce him for never being home, kid grows up seeing his father maybe hour or two a day, and years later regret the day he started.
Only he can deicide......it isn't all roses on the self employment side.